From reading around and doing some basic testing myself:
So the game really does not need anywhere near 8 gigs of RAM to run well (like 4 gigs for the application seems to be enough), it just reserves (or something) too much RAM and may even keep on increasing the reserved amount over time (or maybe it does have a leak additionally).
Once that reaches the limits (or near enough) of your amount of RAM, Windows starts fucking around with the page file.
If you have an HDD it may be preferred over any SSD regardless of page file settings (?) and then you get that thing with massive HDD activity and everything coming to a halt.
If only SSDs are present, then maybe their speed can mitigate the performance issues resulting from this, not sure.
If only HDDs are present, then you're completely fucked.
If you have a mixture, then unplug your HDD and see if that fixes the problem (if you have the relevant stuff on the SSD).
Either way mostly a developer issue, it seems, coupled with certain Windows behavior (although I'm not sure if that part can be fixed with some tweaks of the settings).
This game should be CPU bound in single player due to the A.I. even on relatively old systems, and their CPU requirements do seem to reflect that.
Should run like a dream (big sieges with intense A.I. stuff aside) once they finally fix this. Just disappointing to see such a big issue after such a long dev time.
One thing that should absolutely NOT be necessary is buying more than 8 gigs of RAM, at least not for this game... Even if 16 is a better baseline for win 10 users.
You also should not actually need an SSD at all (unless you want the faster loading times). 8 gigs of RAM and an HDD can run this fine if the bug is fixed, I think.